I would not attempt this because of the difficulty drilling / milling a straight hole. As well, doing so would certainly void the warranty.
Alternatively, one could attach weight to the lower tiller bolt in a number of ways. I have tried a number of weight configurations and like weight at the lower position best. The "pendulum" effect seems to be very good for aiming.
Anything is possible, but that would ruin the risers good looks and possibly ruin the riser itself.. I have been shooting a Black Magic for a couple weeks now and wouldn't dream of doing anything like this.to it.
I had Martin do that to a Mamba I had.
It worked fine but for some reason they didn't glue the insert and the bottom limb split about two weeks after my getting it.
Martin replaced the bow, which is how I came to of owned two Mambas.
I'd do it again on a riser alone, but not a one piece bow.